I think formatting the dates should be extracted to a separate task.

Investigate it further, and it turns out that Wicket i18n support is quite elegant. And we can implement this feature modifying three places:

1. Modify date-picker.js to detect browser language automatically and configure flatpickr:

  var language = navigator.language;
  var index = language.indexOf('-');
  if (index != -1)
    language = language.substring(0, index);
  var calendar = flatpickr($input[0], {dateFormat: "Y-m-d", "allowInput": true, "locale": language});

1. Add a new dependency to dependency list of DatePickerResourceReference:

		dependencies.add(JavaScriptHeaderItem.forReference(new JavaScriptResourceReference(DatePickerResourceReference.class, "flatpickr-locale.js")));

With this code, Wicket will load flatpickr-locale_<language>.js automatically based on browser language, and fallback to flatpickr-locale.js if relevant language file does not exist

1. Add an empty fallback file flatpickr-locale.js under date picker directory. Move various flatpickr language files into the same directory, and rename them as flatpickr-locale_<language>.js. For instance: rename de.js as flatpickr-locale_de.js

Please ignore suggested change #1 above. Your current code of setting flatpickr locale is more robust:

 var locale = (navigator.language || 'en').toLowerCase();
 var shortLocale = locale.substr(0, 2);
 if (flatpickr.l10ns[shortLocale]) {
   flatpickr.localize(flatpickr.l10ns[shortLocale]);
 }

Investigate it further, and it turns out that Wicket i18n support is quite elegant.

For some reasons I ended up in the 6.x docs. So OneDev uses Wicket 7?

With this code, Wicket will load flatpickr-locale_.js automatically based on browser language, and fallback to flatpickr-locale.js if relevant language file does not exist

Done

Add an empty fallback file flatpickr-locale.js under date picker

And done

ame directory, and rename them as flatpickr-locale_.js. For instance: rename de.js as flatpickr-locale_de.js

I would have placed it in a subfolder since the many files are kind of confusing.